„Exposure control“ is the selection of a specific combination of exposure time, aperture and ISO sensitivity that provides an exposure matching the result of the exposure metering and a possible additional exposure compensation.
There are four types of exposure control for the choice of exposure time and aperture:
Program Auto: The camera chooses both exposure time and aperture.
This can be done using a generic mapping or optimized for certain shooting situations if you use so called scene modes, e.g. towards shortest possible exposure times for sports, small f-numbers for small depth-of-field for portraits etc.
This mode is either named with a symbol P or digital cameras apply it as part of a fully automated operation mode or scene programs („AUTO“ or „SCENE“ in the example on the right)..
But this does not mean that you cannot take any influence: At advanced cameras you can shift the combination of exposure time and aperture manually if you do not like the suggested values.
Shutter priority: The camera chooses an aperture for an exposure time that you have selected.
Nikon and Sony use an S for this mode, Canon Tv („time value“).
If the exposure time you have selected would require an aperture that your lens cannot provide, your picture would get under- or overexposed.
Aperture priority: The camera chooses an exposure time for the aperture that you have selected.
Nikon and Sony use an A for this mode, Canon Av („aperture value“).
Accidental under- or overexposures will normally not happen in this mode as the range of exposure times is large enough to find a suitable one for your aperture.
Manual control: The camera sets neither exposure time nor aperture automatically. You have to set both yourself however the camera will normally display an indication how much the exposure you have set is brighter or darker than the automatically selected one.
The common labeling for this mode is easy to remember, an M.
The way to set the related ISO sensitivity depends on your camera and possibly settings on your camera. It can be
- a value you have chosen
- an automatically determined value.
The logic how a digital camera will adjust the ISO sensitivity automatically may also differ from camera to camera. Common options are that the ISO sensitivity is increased when the exposure time gets too long or that it is integrated in the rules of a scene mode.